Conventional actuators, sometimes referred to as “motors”, have a movably supported member, and a coil. When a current is passed through the coil, a motive force is exerted on the member. A control circuit is coupled to the coil in order to controllably supply current to the coil. One example of such an arrangement is found in a hard disk drive, where the movable member of the actuator supports a read/write head adjacent a rotating magnetic disk for approximately radial movement of the head relative to the disk. There are situations in which it is desirable to move the member to one end of its path of travel at a predetermined velocity which is less than its maximum velocity. An example of such a situation is a power failure. In such a situation, it is desirable to move the member to a parking location, where it is held against potentially damaging movement which could occur if the member were not so parked. The movement of the member to the parking location is commonly referred to as a retract of the member.
When a current is applied to the coil of the actuator, the member is subjected to a force tending to accelerate the member at a rate defined by the magnitude of the current, and in a direction defined by the polarity of the current. Consequently, in order to accelerate or decelerate the member until it is moving at a desired velocity and in a desired direction, it is important to know the actual direction and velocity of the member. In this regard, it is known that the back-EMF voltage on the coil of the actuator is representative of the velocity and direction of movement of the member. Specifically, the following relationship applies to actuators:VM=IM*RM+Keωwhere:                VM=voltage across actuator (motor),        IM=current through actuator,        RM=internal resistance of actuator,        Ke=torque constant of actuator, and        ω=velocity of actuator.        
The term, Keω, represents the back-EMF of the actuator coil.
Apparatus have been provided that control such actuators by providing a drive current to the coil of the actuator in response to the provision of a target speed voltage signal having a voltage corresponding to the target speed of the moveable member. For example, commonly assigned U.S. Pat. No. 6,040,671, entitled “CONSTANT VELOCITY CONTROL FOR AN ACTUATOR USING SAMPLED BACK EMF CONTROL,” and commonly assigned U.S. Pat. No. 6,184,645 entitled “VOLTAGE MODE DRIVE FOR CONTROL CIRCUIT FOR AN ACTUATOR USING SAMPLED BACK EMF CONTROL” discloses such an apparatus. However, such apparatus does not lend itself readily to providing such control in cases where forces can accelerate the actuator in the same direction driven by the control system.
There is desired a control for an actuator when there is any force accelerating the actuator in any direction.